Use Mix.PubSub

## Description

Newer versions of Elixir come with [Mix.PubSub](https://github.com/elixir-lang/elixir/blob/426a4297c9b9cf31ce41604be0435bafe5d9175d/lib/mix/lib/mix/pubsub.ex#L4) which allows other compiler instances to publish messages to each other when they do things.

This is useful if you are writing code and one process compilers and it removes a module and the other module needs to know about it in order to remove it from the runtime. Or, it compilers new code and you need to load the the module into the runtime.